The invention relates to the determination of frequency instability noise from a source and, more particularly, to a method and associated device for determining frequency instability noise from a phase alignment loop.
According to the prior art, the measurement of frequency instability noise from a source, commonly called xe2x80x9cjitterxe2x80x9d, is carried out by directly analyzing the signal supplied by the source. Two types of methods are commonly used.
A first method includes the direct measurement of the time jitter of the signal supplied by the source. An apparatus that allows time measurements is then used. In the case where the time jitter to be measured is of a low value (for example, of the order of a few tens of picoseconds), it is necessary to use a highly accurate measuring apparatus, which is therefore expensive. Additionally, it is also necessary to make a large number of measurements, which means that the total duration of the time jitter measurement is relatively long.
A second method includes using digital measurements of the output signal from the source using a purely digital tester. This type of measurement involves the processing of the large amount of data and, consequently, an overall test time which is long. Furthermore, the accuracy is limited by the resolution of the tester.
An object of the invention is to avoid the disadvantages mentioned above. Thus, the invention provides a method for measuring frequency instability noise from a source, including the following steps: a first step of measuring the signal to noise ratio SRN1 at the output from a circuit having an input to receive a frequency input signal ft, and an output to supply an output signal, and a sequencing input to receive a clock signal, the clock signal being a reference signal whose frequency instability noise level is less than the frequency instability noise level of the source; and a second step of determining the signal to noise ratio SRN2 at the output from the circuit, the input signal being the frequency input signal ft and the clock signal being a signal coming from the source. The method further includes a step of calculating the frequency instability noise xcex94TRMS from SRN1, SRN2 and ft so that:       Δ    ⁢          xe2x80x83        ⁢          T      RMS        =                    (                              10                                          -                SNR2                            10                                -                      10                                          -                SNR1                            10                                      )                    1        2                    2      ⁢      Π      ⁢              xe2x80x83            ⁢      ft      
The invention also provides a device for measuring the frequency instability noise from a source supplying a signal. The measuring device includes a circuit having an input to receive an input signal, an output to supply an output signal and a sequencing input to receive the clock signal. The device also includes a multiplexer having a first input, a second input and an output, the first input being connected to a reference signal whose frequency instability noise level is less than the frequency instability noise level of the signal supplied by the source, the second input being connected to the signal supplied by the source and the output being connected to the sequencing input of the circuit.
According to the preferred embodiment of the invention, the output circuit from which the measurements of the signal to noise ratio are carried out is an analog/digital converter which receives at its input an analog signal of frequency ft and the source is a phase alignment loop.
Advantageously, the invention eliminates the necessity to directly measure the noise at the source output.